Saturday, January 05, 2008

I've heard this malaprop often, and again recently: Guinness Gas described as nitrous.

So-called Guinness gas is ablend of 60-75% nitrogen(N) and 25-40% carbon dioxide (CO2). Originally developed by Guinness in part to imitate the naturally cascading bubbles of cask ale, this mixture is now considered by many to be di riguer for dispensing stouts.[Stout, Michel J. Lewis, Brewers Publications, 1995, p. 119]It's ironic to note, however, that nitrogen is not natural to the brewing process. It must be added in.

If beer were indeed served via nitrous, rather than this nitrogen gas blend, the results might be quite hilarious. Nitrous is shorthand for nitrous oxide (N2O) — laughing gas, an anesthetic used during dental procedures. It's most definitely NOT in beer!

Beer refrigerators are often placed far from the actual bar where the serving taps are located. Greater pressure is therefore needed to move the beer that greater distance. But using higher pressured CO2 to push the beer a greater distance will create a lot of foaming across the tap.

To get around this, many draft systems employ a nitrogen/CO2 blend. Nitrogen is much less soluble in beer than carbon dioxide, so a Guinness blend can push the beer at greater pressure for greater distances without causing foam across the tap.

There is a problem. Many bars make the common mistake of placing any stout on a Guinness gas line.

Stouts, because of some of the grains used in their recipe grists tend to contain a higher proportion of middle-weight proteins. These promote head retention. But stouts do not naturally contain nitrogen. That is not part of the brewing process. It is an artificial injection during packaging.

It is nitrogenated beers — such as Guinness — which will display the cascading waterfall effect when poured on a Guinness gas line (or cask ales, naturally without any gas). Pushing the beer through very small holes in a plate in a special tap, the nitrogen and CO2 break out of solution in very small, and thus stable, bubbles. But since nitrogen is insoluble, it stays in the bubbles, as opposed to CO2 which tends to break out — hence the tight thick head.

(It's interesting to note that draft Guinness is very lightly carbonated, in fact to no more than one volume of CO2, the level of most cask beers. The rest of the bubbles are nitrogen-derived, applied in the keg under 25 pounds per square inch.)

If a stout has not been nitrogenated — that is if it has been produced without artificially adding nitrogen — and if it's placed on a Guinness gas line, the result will be a lifeless, flat, beer. With so little CO2 in the mix, the CO2 in the keg will evolve out of the beer itself and into the headspace of the keg. The beer will dramatically lose its carbonation — i.e., go flat. The totalpressure of the blend doesn't matter as much as the percentages of the gas content in the mix.

There is a simple expedient however.

By reversing this gas mixture — that is, by using a blend of 60-75% CO2 and 25-40% nitrogen — there will be enough nitrogen to push the beer a long distance — without foaming — and enough of CO2 in the mix to prevent the beer from going flat.

Guinness gas is often referred to as mixed-gas dispense. This is somewhat imprecise, because there has been another mixed-gas blend used for years to push beer long distances: CO2 ... and air!

A simple air compressor would produce the additional pressure above and beyond the standard 15 psi at which most kegs are CO2 pressurized.

That's fine and dandy if the beer is to be consumed rapidly (similar to using a picnic pump to dispense a keg at a tailgating party). But if the beer sits for any length of time (even overnight), you've got nasty stuff.

Here's a deep dark secret: a few bars still do this ... as do many sports arenas.

That's usually not a problem with North American Industrial Lagers (N.A.I.L.s) and their ilk, since they are sold in high enough quantities at sporting and music events. But any craft beer served like this will suffer deleterious flavor effects. It would be like leaving a glass of beer open and sitting on your table for several days before drinking it.

2 comments:

very educational. thank you. I have always loved getting nitro tap stouts at bars, and as a homebrewer, have been trying to learn about applying it to my own set up. considering all the misinformation out there, this has been really helpful.